Recently, semiconductor laser diodes (hereinafter, referred to as “semiconductor laser”) have come to be widely used in a variety of image forming apparatuses using electrophotography, for example, laser printers and digital copiers.
However, the semiconductor laser degrades due to various factors. More specifically, degradation of the semiconductor laser may arise due to crystal defect within several hours. Further, the degradation of the semiconductor laser may arise gradually by being driven at high temperature for a long time.
Further, the degradation of the semiconductor laser may arise due to catastrophic optical damage (COD) or an electrostatic discharge (ESD) during assembly of the laser printers and the digital copiers.
When the semiconductor laser degrades, a larger drive current is required to emit light at a predetermined intensity (quantity of light). When the drive current is increased excessively and power consumption exceeds a rated value, the semiconductor laser may degrade due to heat generated by the power and cause trouble. Further, when a large current beyond the assumed value flows from a semiconductor laser drive device, there is also a possibility of a defect arising due to melting of metal wiring.
JP-2005-32798-A discloses a semiconductor laser drive device that can detect such a degradation of the semiconductor laser. FIGS. 1 and 2 represent a circuit diagram of the semiconductor laser drive device disclosed in JP-2005-32798-A.
In one example, the semiconductor laser drive device shown in FIG. 1 outputs a current Im attenuated from a drive current Iop at a fixed ratio. This output current Im is converted to a voltage Vm. The voltage Vm is compared with a fixed voltage (half of a power-supply voltage) VCC½ by a comparator. When the voltage Vm exceeds the fixed voltage VCC½ (Vm>Vcc/2), a signal Serr indicating abnormality of the value of drive current Iop is output to an image controller provided at a previous stage of the semiconductor laser drive device. Accordingly, the image controller can recognize the abnormality based on the signal Serr indicating the abnormality.
However, when the value of drive current Iop is small and below a predetermined current value, the signal indicating abnormality may not be output in the semiconductor laser drive device shown in FIG. 1.
In another example, a semiconductor laser drive device shown in FIG. 2 outputs the current Im attenuated from the drive current Iop at a fixed ratio. This current Im is converted to the voltage Vm representing the value of the drive current Iop by a current to a voltage conversion circuit R2, and is output to an image controller provided at the previous stage of the semiconductor laser drive device. Accordingly, the image controller can recognize the abnormality based on the voltage Vm representing the value of the drive current Iop.
However, the semiconductor laser drive device shown in FIG. 2 outputs an analog value, making it necessary to mount an analog-to-digital (A/D) converter that converts an analog value to a digital value on the circuit board or have a built-in circuit in the image controller because the image controller cannot perform data processing with the analog value directly.
Accordingly, the number of parts on the circuit board of the image controller increases. Consequently, the configuration of the image controller becomes complicated and the chip area of the image controller expands, resulting in a cost penalty.